arXiv subjects

Ivan Vitev

Publications and source records attributed to Ivan Vitev.

At least 73 records · Page 4Linked to original sources

arXiv2016

Jet substructure using semi-inclusive jet functions within SCET

We propose a new method to evaluate jet substructure observables in inclusive jet measurements, based upon semi-inclusive jet functions in the framework of Soft Collinear Effective Theory (SCET). As a first example, we consider the jet fragmentation function, where a hadron $h$ is identified inside a fully reconstructed jet. We introduce a new semi-inclusive fragmenting jet function ${\mathcal G}^h_i(z= ω_J/ω,z_h=ω_h/ω_J,ω_J, R,μ)$, which depends on the jet radius $R$ and the large light-cone momenta of the parton `$i$' initiating the jet ($ω$), the jet ($ω_J$), and the hadron $h$ ($ω_h$). The jet fragmentation function can then be expressed as a semi-inclusive observable, in the spirit of actual experimental measurements, rather than as an exclusive one. We demonstrate the consistency of the effective field theory treatment and standard perturbative QCD calculations of this observable at next-to-leading order (NLO). The renormalization group (RG) equation for the semi-inclusive fragmenting jet function ${\mathcal G}_i^h(z,z_h, ω_J, R,μ)$ are also derived and shown to follow exactly the usual timelike DGLAP evolution equations for fragmentation functions. The newly obtained RG equations can be used to perform the resummation of single logarithms of the jet radius parameter $R$ up to next-to-leading logarithmic (NLL$_R$) accuracy. In combination with the fixed NLO calculation, we obtain NLO+NLL$_R$ results for the hadron distribution inside the jet. We present numerical results for $pp\to(\mathrm{jet}\,h)X$ in the new framework, and find excellent agreement with existing LHC experimental data.

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arXiv2016

The semi-inclusive jet function in SCET and small radius resummation for inclusive jet production

We introduce a new kind of jet function: the semi-inclusive jet function $J_i(z, ω_J, μ)$, which describes how a parton $i$ is transformed into a jet with a jet radius $R$ and energy fraction $z = ω_J/ω$, with $ω_J$ and $ω$ being the large light-cone momentum component of the jet and the corresponding parton $i$ that initiates the jet, respectively. Within the framework of Soft Collinear Effective Theory (SCET) we calculate both $J_q(z, ω_J, μ)$ and $J_g(z, ω_J, μ)$ to the next-to-leading order (NLO) for cone and anti-k$_{\rm T}$ algorithms. We demonstrate that the renormalization group (RG) equations for $J_i(z, ω_J, μ)$ follow exactly the usual DGLAP evolution, which can be used to perform the $\ln R$ resummation for {\it inclusive} jet cross sections with a small jet radius $R$. We clarify the difference between our RG equations for $J_i(z, ω_J, μ)$ and those for the so-called unmeasured jet functions $J_i(ω_J, μ)$, widely used in SCET for {\it exclusive} jet production. Finally, we present applications of the new semi-inclusive jet functions to inclusive jet production in $e^+e^-$ and $pp$ collisions. We demonstrate that single inclusive jet production in these collisions shares the same short-distance hard functions as single inclusive hadron production, with only the fragmentation functions $D_i^h(z, μ)$ replaced by $J_i(z, ω_J, μ)$. This can facilitate more efficient higher-order analytical computations of jet cross sections. We further match our $\ln R$ resummation at both LL$_{R}$ and NLL$_{R}$ to fixed NLO results and present the phenomenological implications for single inclusive jet production at the LHC.

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arXiv2016

Effective field theory approach to open heavy flavor production in heavy-ion collisions

We develop a version of Soft Collinear Effective Theory (SCET) which includes finite quark masses, as well as Glauber gluons that describe the interaction of collinear partons with QCD matter. In the framework of this new effective field theory, labeled SCET$_{\mathrm{M,G}}$, we derive the massive splitting functions in the vacuum and the QCD medium for the processes $Q\to Qg$, $Q\to gQ$ and $g\to Q\bar Q$. The numerical effects due to finite quark masses are sizable and our results are consistent with the traditional approach to parton energy loss in the soft gluon emission limit. In addition, we present a new framework for including the medium-induced full splitting functions consistent with next-to-leading order calculations in QCD for inclusive hadron production. Finally, we show numerical results for the suppression of $D$- and $B$-mesons in heavy ion collisions at $\sqrt{s_{\mathrm{NN}}}=5.02$ TeV and 2.76 TeV and compare to available data from the LHC.

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arXiv2016

Semi-inclusive jet cross sections within SCET

We review the definition of semi-inclusive jet functions within Soft Collinear Effective Theory (SCET) and their application to inclusive jet cross sections. As an example, we consider both the inclusive production of jets and the jet fragmentation function in proton-proton collisions. The semi-inclusive jet functions satisfy renormalization group (RG) equations which take the form of standard timelike DGLAP evolution equations, analogous to collinear fragmentation functions. By solving these RG equations, the resummation of potentially large single logarithms $(α_s \ln R)^n$ can be achieved. We present numerical results at NLO+NLL$_R$ accuracy and compare to existing data from the LHC.

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arXiv2016

Probing the hardest branching of jets in heavy ion collisions

We present the first calculation of the momentum sharing and angular separation distributions between the leading subjets inside a reconstructed jet in heavy ion collisions. These observables are directly sensitive to the hardest branching in the process of jet formation and are, therefore, ideal for studying the early stage of the in-medium parton shower evolution. The modification of the momentum sharing and angular separation distributions in lead-lead relative to proton-proton collisions is evaluated using the leading-order medium-induced splitting functions obtained in the framework of soft-collinear effective theory with Glauber gluon interactions. Qualitative and in most cases quantitative agreement between theory and preliminary CMS measurements suggests that the parton shower in heavy ion collisions can be dramatically modified early in the branching history. We propose a new measurement which will illuminate the angular distribution of the hardest branching within jets in heavy ion collisions.

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arXiv2016

Jet fragmentation functions in proton-proton collisions using soft-collinear effective theory

The jet fragmentation function describes the longitudinal momentum distribution of hadrons inside a reconstructed jet. We study the jet fragmentation function in proton-proton collisions in the framework of soft-collinear effective theory (SCET). We find that, up to power corrections, the jet fragmentation function can be expressed as the ratio of the fragmenting jet function and the unmeasured jet function. Using renormalization group techniques, we are able to resum large logarithms of jet radii R in the perturbative expansion of the cross section. We use our theoretical formalism to describe the jet fragmentation functions for light hadron and heavy meson production measured at the Large Hadron Collider (LHC). Our calculations agree very well with the experimental data for the light hadron production. On the other hand, although our calculations for the heavy meson production inside jets are consistent with the PYTHIA simulation, they fail to describe the LHC data. We find that the jet fragmentation function for heavy meson production is very sensitive to the gluon-to-heavy-meson fragmentation function.

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arXiv2016

Towards the understanding of jet shapes and cross sections in heavy ion collisions using soft-collinear effective theory

We calculate the jet shape and the jet cross section in heavy ion collisions using soft-collinear effective theory (SCET) and its extension with Glauber gluon interactions in the medium (SCET$_{\rm G}$). We use the previously developed framework to systematically resum the jet shape at next-to-leading logarithmic accuracy, and we consistently include the medium modification by incorporating the leading order medium-induced splitting functions. The calculation provides, for the first time, a quantitative understanding of the jet shape modification measurement in lead-lead collisions at $\sqrt{s_{\rm NN}}=2.76$ TeV at the LHC. The inclusive jet suppression is also calculated within the same framework beyond the traditional concept of parton energy loss, and the dependence on the centrality, the jet radius and the jet kinematics is examined. In the end we present predictions for the anticipated jet shape and cross section measurements in lead-lead collisions at $\sqrt{s_{\rm NN}}\approx5.1$ TeV at the LHC.

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arXiv2016

Photon-tagged and B-meson-tagged b-jet production at the LHC

Tagged jet measurements in high energy hadronic and nuclear reactions provide constraints on the energy and parton flavor origin of the parton shower that recoils against the tagging particle. Such additional insight can be especially beneficial in illuminating the mechanisms of heavy flavor production in proton-proton collisions at the LHC and their modification in the heavy ion environment, which are not fully understood. With this motivation, we present theoretical results for isolated-photon-tagged and B-meson-tagged b-jet production at center-of-mass energy 5.1 TeV for comparison to the upcoming lead-lead data. We find that photon-tagged b-jets exhibit smaller momentum imbalance shift in nuclear matter, and correspondingly smaller energy loss, than photon-tagged light flavor jets. Our results show that B-meson tagging is most effective in ensuring that the dominant fraction of recoiling jets originate from prompt b-quarks. Interestingly, in this channel the large suppression of the cross section is not accompanied by a significant momentum imbalance shift.

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arXiv2016

Initial-state cold nuclear matter energy loss effects on inclusive jet production in p+A collisions at RHIC and LHC

Recent measurements of the centrality and rapidity dependence of single inclusive jet production in p+Pb collisions at the LHC have revealed large and non-trivial nuclear modification of the production cross section for this process. In this paper, we explore to what extent such nuclear modification can be understood by the framework of standard cold nuclear matter effects, in particular initial-state cold nuclear matter energy loss. We demonstrate quantitatively that theoretical calculations which include medium-induced radiative corrections can describe rather reasonably the attenuation of the jet production yields in the large transverse momentum region in d+Au collisions at RHIC and p+Pb collisions at the LHC for central to semi-central collisions. We further show that the observed scaling behavior of the nuclear modification factor as a function of the total jet energy $p_T\, {\cosh} (y)$ for various rapidity intervals has a natural explanation in the picture of cold nuclear matter energy loss. On the other hand, the observed enhancement in peripheral collisions is not described in this picture and could have a different origin.

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arXiv2016

Jet Quenching from QCD Evolution

Recent advances in soft-collinear effective theory with Glauber gluons have led to the development of a new method that gives a unified description of inclusive hadron production in reactions with nucleons and heavy nuclei. We show how this approach, based on the generalization of the DGLAP evolution equations to include final-state medium-induced parton showers, can be combined with initial-state effects for applications to jet quenching phenomenology. We demonstrate that the traditional parton energy loss calculations can be regarded as a special soft-gluon emission limit of the general QCD evolution framework. We present phenomenological comparison of the SCET$_{\rm G}$-based results on the suppression of inclusive charged hadron and neutral pion production in $\sqrt{s_{NN}}=2.76$ TeV lead-lead collisions at the Large Hadron Collider to experimental data. We also show theoretical predictions for the upcoming $\sqrt{s_{NN}} \simeq 5.1$ TeV Pb+Pb run at the LHC.

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arXiv2016

Spin asymmetries for vector boson production in polarized p+p collisions

We study the cross section for vector boson ($W^{\pm}/Z^0/γ^*$) production in polarized nucleon-nucleon collisions for low transverse momentum of the observed vector boson. For the case where one measures the transverse momentum and azimuthal angle of the vector bosons, we present the cross sections and the associated spin asymmetries in terms of transverse momentum dependent parton distribution functions (TMDs) at tree level within the TMD factorization formalism. To assess the feasibility of experimental measurements, we estimate the spin asymmetries for $W^{\pm}/Z^0$ boson production in polarized proton-proton collisions at the Relativistic Heavy Ion Collider (RHIC) by using current knowledge of the relevant TMDs. We find that some of these asymmetries can be sizable if the suppression effect from TMD evolution is not too strong. The $W$ program at RHIC can, thus, test and constrain spin theory by providing unique information on the universality properties of TMDs, TMD evolution, and the nucleon structure. For example, the single transverse spin asymmetries could be used to probe the well-known Sivers function $f_{1T}^{\perp q}$, as well as the transversal helicity distribution $g_{1T}^{q}$ via the parity-violating nature of $W$ production.

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arXiv2015

Soft-collinear effective theory for hadronic and nuclear collisions: The evolution of jet quenching from RHIC to the highest LHC energies

In the framework of soft-collinear effective theory with Glauber gluons, results and predictions for inclusive hadron suppression, based upon in-medium parton shower evolution, are presented for Au+Au and Pb+Pb collisions at RHIC and LHC energies $\sqrt{s}=200$ AGeV and $\sqrt{s}=2.76, \, 5.1$ ATeV, respectively. The $\rm SCET_G$ medium-induced splitting kernels are further implemented to evaluate the attenuation of reconstructed jet cross in such reactions and to examine their centrality and radius $R$ dependence. Building upon a previously developed method to systematically resum the jet shape at next-to-leading logarithmic accuracy, a quantitative understanding of the jet shape modification measurement in Pb+Pb collisions at $\sqrt{s}=2.76$ ATeV at the LHC can be achieved. Predictions for photon-tagged jet cross sections and shapes, that can shed light on the parton flavor dependence of in-medium parton shower modification, are also given.

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arXiv2015

Next-to-leading order transverse momentum-weighted Sivers asymmetry in semi-inclusive deep inelastic scattering: the role of the three-gluon correlator

We study the Sivers asymmetry in semi-inclusive hadron production in deep inelastic scattering. We concentrate on the contribution from the photon-gluon fusion channel at ${\cal O}(α_{em}^2 α_s)$, where three-gluon correlation functions play a major role within the twist-3 collinear factorization formalism. We establish the correspondence between such a formalism with three gluon correlation functions and the usual transverse momentum dependent (TMD) factorization formalism at moderate hadron transverse momenta. We derive the coefficient functions used in the usual TMD evolution formalism related to the quark Sivers function expansion in terms of the three-gluon correlation functions. We further perform the next-to-leading order calculation for the transverse-momentum-weighted spin-dependent differential cross section, and identify the off-diagonal contribution from the three-gluon correlation functions to the QCD collinear evolution of the twist-3 Qiu-Sterman function.

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arXiv2015

Initial-state splitting kernels in cold nuclear matter

We derive medium-induced splitting kernels for energetic partons that undergo interactions in dense QCD matter before a hard-scattering event at large momentum transfer $Q^2$. Working in the framework of the effective theory ${\rm SCET}_{\rm G}\,$, we compute the splitting kernels beyond the soft gluon approximation. We present numerical studies that compare our new results with previous findings. We expect the full medium-induced splitting kernels to be most relevant for the extension of initial-state cold nuclear matter energy loss phenomenology in both p+A and A+A collisions.

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arXiv2015

A Study of Quasi-parton Distribution Functions in the Diquark Spectator Model

To facilitate lattice QCD calculations of nucleon structute, a set of quasi-parton distributions were recently introduced. These quasi-PDFs were shown to reduce to standard PDFs when the nucleon is boosted to high energies, $P_z\rightarrow \infty$. Since taking such limit is not feasible in lattice simulations, it is essential to provide guidance for what values of $P_z$ the quasi-PDFs are good approximations of standard PDFs. Within the framework of the spectator diquark model, we evaluate both the up and down quarks' quasi-PDFs and standard PDFs for all leading-twist distributions (unpolarized distribution $f_1$, helicity distribution $g_1$, and transversity distribution $h_1$). We find that, for intermediate parton momentum fractions $x$, quasi-PDFs are good approximations to standard PDFs (within $20-30\%$) when $P_z\gtrsim 1.5-2$ GeV. On the other hand, for large $x\sim 1$ much larger $P_z > 4$ GeV is necessary to obtain a satisfactory agreement between the two sets. We further find that the Soffer positivity bound does not hold in general for quasi-PDFs.

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arXiv2015

Quenching of inclusive and tagged b-jets at the LHC

We present theoretical predictions for the nuclear-induced attenuation of the differential cross sections for inclusive and tagged b-jet production in heavy ion collisions at the LHC. We find that for inclusive b-jet production at high transverse momentum the mass effects are negligible, and that the attenuation is comparable to the one observed for light jets. On the other hand, for isolated-photon and B-meson-tagged b-jets the sample of events with heavy quarks produced at the early stages of the collision is greatly enhanced. Thus, these tagged b-jet final-states have a much more direct connection to the physics of b-quark energy loss. We present theoretical predictions for the quenching of such tagged b-jet events at the LHC and the QGP-induced modification of the related momentum imbalance and asymmetry. We demonstrate that these tagged processes can be used to accurately study the physics of heavy quark production and propagation in dense QCD matter.

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arXiv2015

Quasi-parton distribution functions: a study in the diquark spectator model

A set of quasi-parton distribution functions (quasi-PDFs) have been recently proposed by Ji. Defined as the matrix elements of equal-time spatial correlations, they can be computed on the lattice and should reduce to the standard PDFs when the proton momentum $P_z$ is very large. Since taking the $P_z\to \infty$ limit is not feasible in lattice simulations, it is essential to provide guidance for what values of $P_z$ the quasi-PDFs are good approximations of standard PDFs. Within the framework of the spectator diquark model, we evaluate both the up and down quarks' quasi-PDFs and standard PDFs for all leading-twist distributions (unpolarized distribution $f_1$, helicity distribution $g_1$, and transversity distribution $h_1$). We find that, for intermediate parton momentum fractions $x$, quasi-PDFs are good approximations to standard PDFs (within $20-30%$) when $P_z\gtrsim 1.5-2$ GeV. On the other hand, for large $x\sim 1$ much larger $P_z > 4$ GeV is necessary to obtain a satisfactory agreement between the two sets. We further test the Soffer positivity bound, and find that it does not hold in general for quasi-PDFs.

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arXiv2014

Jet Shape Resummation Using Soft-Collinear Effective Theory

The jet shape is a classic jet substructure observable that probes the average transverse energy profile inside a reconstructed jet. The studies of jet shapes in proton-proton collisions have served as precision tests of perturbative Quantum Chromodynamics (QCD). They have also recently become the baseline for studying the in-medium modification of parton showers in ultra-relativistic nucleus-nucleus collisions. The jet shape is a function of two angular parameters $R$ and $r$, which can be at hierarchical scales. Its calculation suffers from large logarithms of the ratio between the two scales, and these phase space logarithms can conveniently be resummed in the framework of soft-collinear effective theory (SCET). We find that, up to power corrections, the integral jet shape can be expressed in a factorized form which involves only the ratio between two jet energy functions. Resummation is performed at next-to-leading logarithmic order using renormalization-group evolution techniques. Comparisons to jet shape measurements at the LHC are presented to verify the dominant role of the collinear parton shower and to identify the kinematic region in which power-suppressed soft modes and non-perturbative effects may play a role.

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